Coarse wavelength division multiplexing (CWDM) is a relatively new application of conventional wavelength division multiplexing (WDM) that is being used in short haul and metro networks. The more well-known long haul and ultra-long haul dense wavelength division multiplexing (DWDM) techniques that became popular in the mid to late 1990s allow for many wavelengths to be transmitting on a single fiber, but these DWDM techniques are expensive. The expense is a result of tighter tolerances required for many components of an overall communication system as more wavelengths are multiplexed onto a single fiber. To avoid cross talk between adjacent wavelengths, lasers must be controlled to maintain a transmission wavelength line width in an extremely limited wavelength range. This is done through the use of TEC (Thermo-electric cooling) devices which can be expensive. DWDM bandpass filters must also have tightly controlled specifications to ensure adjacent wavelengths are suppressed to a satisfactory level and do not interfere with a specific bandpass channel. These control systems and filters are expensive due to research and development costs and the difficulty involved with their manufacture.
One of the goals of metro networks is to provide a communication network for a localized area at a minimum cost. One simple manner to achieve this is to use fewer wavelengths on a single fiber. In this way specifications of the system components can be eased. Laser transmission wavelengths are allowed to vary and WDM bandpass filters can have wider passband bandwidths. If looser tolerances are acceptable, the cost of the overall system is reduced.
Typically, signals carried on short haul and metro networks have not required signal amplification as network links were less than 50 kms. However, as short haul and metro networks become more popular and find greater use, they are growing in size to include parts of the access network to where amplification of the signals carried by the networks is a requirement.
This creates the dilemma of how to provide cost effective optical signal amplification for short haul and metro network markets. Conventional C-band (typically 1528 nm to 1565 nm) plus L-band (typically 1570 nm to 1605 nm) optical amplifiers of the type used in DWDM applications are more expensive than metro network markets wish to pay.
Currently, semi-conductor amplifier (SOA) technology is capable of providing optical signal amplification for blue-band CWDM applications. Blue-band usually covers the wavelengths 1470, 1490, 1510, 1530 nm, the blue band including wavelength's in the S-band and the C-band. However, SOA technology has not yet been able to provide amplification for red-band CWDM applications in the same cost effective manner. Red-band covers the wavelengths 1550, 1570, 1590, 1610 nm, the red band including wavelengths in the C-band and the L band.